1. Field of the Invention
The present invention generally relates to image forming apparatuses and, in particular, to an image forming apparatus having a recording head that ejects liquid droplets.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, and a multi-task machine having plural such functions, a known ink jet recording apparatus of a liquid ejection recording type uses a recording head that ejects, for example, ink liquid droplets. The image forming apparatus of this type ejects ink droplets onto a sheet during conveyance from the recording head to perform image formation (used synonymously with recording, printing, and imaging). Examples of an image forming apparatus include a serial-type image forming apparatus in which a recording head ejects liquid droplets to form an image while moving in a main scanning direction and a line-type image forming apparatus using a line-type head in which a recording head ejects liquid droplets to form an image without moving.
Note that in the present invention, an “image forming apparatus” refers to an apparatus that ejects ink droplets onto a medium such as paper, a thread, a fiber, a fabric, leather, metal, a plastic, glass, wood, and a ceramic so as to perform image formation. Further, “image formation” refers to forming not only relevant images such as characters and graphics but also irrelevant images such as patterns on a medium (the image forming apparatus also includes one referred to as a liquid droplet ejection apparatus or a liquid ejection apparatus that merely ejects liquid droplets onto a medium). Further, “ink” is not limited to one as generally called ink but is used as a generic name of various liquids available for image formation such as recording liquid, fixing treatment liquid, DNA samples, and patterning materials. Further, the material of a “sheet” is not limited to paper. That is, the sheet refers to those including an OHP sheet, a fabric, etc., onto which ink droplets are ejected and is used as a generic name of those including a medium to be recorded on, a recording medium, a recording sheet, a recording paper, etc. Further, an “image” is not limited to a two-dimensional image but also refers to an image added to a three-dimensional object and an image obtained by shaping an object into three dimensions.
A known liquid ejection head (a liquid droplet ejection head) used as a recording head includes a piezoelectric-type liquid ejection head and a thermal-type head. The piezoelectric-type liquid ejection head deforms a vibration plate with a piezoelectric actuator or the like and changes the volumes of liquid chambers. Thus, the piezoelectric-type liquid ejection head increases pressure inside the liquid chambers to eject liquid droplets. Further, the thermal-type head is provided with a heat generation body that generates heat when liquid chambers are energized. Thus, in ejecting liquid droplets, the thermal-type head increases pressure inside the liquid chambers with air bubbles produced when the heat generation body generates heat.
Such image forming apparatuses of the liquid ejection type are expected to in the future particularly increase image forming throughput, i.e., accelerate an image forming speed. Therefore, the image forming apparatus employs a method to for supplying ink from a high-capacity ink cartridge (main tank) installed in a main body a sub-tank (also including those referred to as a buffer tank) provided at the upper part of a recording head via a tube. According to the method (tube supply method) for supplying ink using the tube, a carriage part can be reduced in weight and downsized. In addition, the apparatus including a structure system and a driving system can be significantly downsized.
On the other hand, in order to stably perform liquid ejection with the recording head, it is necessary to apply appropriate negative pressure to nozzles. If the negative pressure is too low, liquid runs from the head. On the other hand, if the negative pressure is too high, the liquid ejection becomes unstable or is disabled.
A configuration for generating negative pressure in the ink supply system described above includes a method for connecting the ink cartridge to the recording head in communication with air by a tube and merely arranging the ink cartridge to be below the recording head. In this method, the negative pressure is obtained according to a water head difference between the ink cartridge and the recording head.
However, in this method, the water head difference fluctuates due to the lowering of the liquid surface of ink inside the ink cartridge when the ink is consumed in the ink cartridge. Therefore, the negative pressure inside the head may be gradually increased. With the increase in the negative pressure inside the head, problems such as unstable ink ejection and a difficulty in restoring the nozzles from their clogged state as described above are caused.
Accordingly, in order to maintain a head at constant pressure, Patent Document 1 (JP-A-2005-342961) discloses a configuration for communicating a head with a circulation path that circulates ink inside a sub-tank, detecting pressure inside the head that fluctuates according to the ejection status of the head with a pressure sensor, and controls the circulation pump provided in the circulation path based on a detection result.
However, the configuration disclosed in Patent Document 1 (JP-A-2005-342961) requires complicated control using the pressure sensor to compensate for the fluctuation of a water head difference.